The biggest, earliest feature you get is Form of Dread, that gives you some temp hitpoints, makes you immune to bring Frightened, and anything you land a hit on has to make a save or it becomes Frightened of you until your next turn.

At 6th level, you don't need to eat, drink, or breathe, which is a great utility feature if your DM has a lot of focus on survival aspects. At 10th level, you're immune to Necrotic damage while in your Form of Dread, and at 14th level you can just have your soul literally leave your body to get up to some no-good mischievous deeds.